1. Field of the Invention
This invention relates to a buried pipeline system for transporting cold product such as refrigerated natural gas through frost-susceptible ground or soil. In particular, the invention pertains to a pipeline system for reducing or eliminating frost heaving of the pipeline due primarily to long-term formation of freeze bulbs around the pipeline.
2. Description of the Prior Art
Recent discoveries of vast quantities of natural gas in the Arctic have created a need for very high flow capacity pipeline systems to transport the natural gas from the Arctic to gas consumers located in more temperate regions of the world. It is now generally accepted that one mode of accomplishing this transportation is by the use of buried, refrigerated gas pipelines in which the natural gas is transported in a gaseous state at high pressures and at temperatures below 32.degree. F. (0.degree. C.). A discussion of refrigerated gas pipelines is given by G. King, "The How and Why of Cooling Arctic Gas Pipelines", Parts I and II, Pipeline and Gas Journal (September and October, 1977). Refrigerated gas pipelines will be required to traverse great distances and must be compatible with the various soil conditions which will be encountered.
Substantial portions of these pipelines will be located in regions of permafrost, which has been defined as a permanently frozen layer at variable depth below the earth's surface in frigid regions. Ice may constitute up to 90% of the total volume of permafrost. In general, permafrost, if it remains frozen, has very good structural strength. A. C. Matthews, "Natural Gas Pipeline Design and Construction in Permafrost and Discontinuous Permafrost", SPE 6873 (1977). One of the prime benefits of buried, refrigerated gas pipelines is their compatibility with permafrost. Operation of these pipelines at temperatures below 32.degree. F. prevents degradation of the permafrost. Thus, the structural integrity of the pipeline system may be maintained without the use of pipe insulation or other protective measures.
Other sections of the pipeline must traverse thawed soils which often are frost-susceptible. A frost-susceptible soil is one which is subject to frost heaving when it freezes. Heaving generally occurs when the frost-susceptible soil contains an adequate supply of water and freezing temperatures are present. In that event, lenses of segregated ice tend to form within the soil. Typical frost-susceptible soils are silt, silty sand, and clay. With the exception of seasonal freezing of the upper layer, these frost-susceptible soils normally remain continuously thawed at a temperature above 32.degree. F.
Between areas of continuous permafrost and areas of continuous thawed soils the pipeline will encounter areas of discontinuous permafrost. The pipeline system must also be compatible with this environment. Discontinuous permafrost comprises distinct patches of both marginal permafrost and thawed ground in a random array. By definition, the temperature of marginal permafrost is only slightly below 32.degree. F.
When a refrigerated gas pipeline is incorporated into thawed soil, the cold pipeline becomes a heat sink which removes heat from the surrounding, warmer soil. This causes the formation of freeze bulbs around the pipeline and, potentially, heaving of the pipeline. This process is related to, but may be distinguished from, seasonal freezing of the soil due to seasonal changes in the ambient air temperature. The formation of freeze bulbs is a long term process, requiring perhaps several years to occur. However, frost heave can be a serious problem, because these pipelines may remain in service for decades. The degree of pipeline uplift due to frost heave is not uniform along the pipeline and the resulting differential movement can deform or even rupture the pipeline.
Reducing frost heave by freezing the thawed soil before the installation of the pipeline has been proposed. This may be done by using passive heat extraction devices to remove heat from the soil during the winter and to radiate the heat into the air. See, for example, those devices disclosed in U.S. Pat. No. 4,194,856 issued to Jahns (1980). Alternatively, the soil around the pipeline may be frozen by the use of passive heat extraction devices which are located below the refrigerated gas pipeline and operate continuously to remove heat from the surrounding soil. The pipeline itself is used as a continuously available heat sink. However, systems utilizing passive heat extraction devices may be quite costly. Hundreds of thousands of these units may be required to protect a long pipeline.
Several other methods have been proposed for dealing with the problem of frost heave of refrigerated gas pipelines. A discussion of the problems and current approaches for operating refrigerated gas pipelines in permafrost and thawed soil may be found in the article by A. C. Matthews, noted above. These methods include excavating a trench deeper than that required to accommodate the pipeline and replacing the frost-susceptible soil surrounding the pipeline with heave resistant material. Other proposed solutions include heavily insulating the pipeline, or heating the soil beneath the pipeline in order to prevent formation of the ice lenses, or taking both measures. Overexcavating and surrounding the pipeline with heave resistant material can delay and mitigate frost heave, but will not solve the problem. The same may be said for insulating the pipeline. Heating the soil generally will involve specialized construction techniques, as where individual electric heaters are installed. Careful surveillance and frequent adjustments of heating rates are also required. Further, adequate methods for monitoring pipeline heave are not yet available. These specialized techniques and devices inherently involve very high, possibly prohibitive costs when great lengths of a pipeline system require frost heave protection. Numerous transitions from frozen to thawed ground may be encountered with any major pipeline located in the cold regions of the world. For example, precautions will be taken to protect the Alaska Highway Gas Pipeline from frost heave over at least an 80 mile length. For details, see Oilweek, page 20 (Apr. 17, 1978).